Electrography, which broadly includes the forming a developing of electrostatic image patterns either with or without light, has become a major field of technology. It perhaps is best known through the use of electrophotographic office copying machines. In electrophotographic processes, a uniform electrostatic charge is placed on a photoconductive insulating layer. The layer is then exposed to a light and shadow image to dissipate the charge on the areas of the layer exposed to light. The resulting electrostatic image is developed by depositing a toner powder on the image. The toner powder is only adherently attracted to those areas of the layer which retain a charge so that the toner image corresponds to the electrostatic image when the charging polarity is opposite that of the toner polarity. Conversely, if the toner polarity is the same as the charging polarity, exposed and thus discharged areas of the image can be toned if the potential applied to the toning assembly is higher than that of the exposed areas. The toner image is then transferred to paper to which it is permanently fused with heat.
High quality electrophotographic process color and other types of continuous toned images require use of smooth, high quality paper such as clay coated lithographic paper stock. To fuse process color images on such paper, the toner powder must include a binder with a melt viscosity substantially lower than that of binders for black and white electrophotographic copying. Such viscosity reduction is needed to achieve the desired properties of surface gloss and elimination of light scattering voids in images. At the same time, however, the toner binder must be useful at a low enough input energy or temperature so that the clay coated paper stock does not blister due to the vaporization of water within the paper.
The toner binder must also minimize offset. Offset occurs when some of the toner adheres to the paper, while some remains on the fusing roller and then bonds to the next piece of paper. Resistance to offset is normally achieved by use of a toner binder with high melt cohesive strength or high melt elasticity (e.g., polymeric toner binders which have a high molecular weight or are crosslinked).
Polyesters represent a useful class of toner binders. See, for example, U.S. Pat. No. 4,546,060 to Miskinis, et al., U.S. Pat. NO. 4,758,491 to Alexandrovich et. al., and U.S. Pat. No. 4,866,158. Amongst these, polyesters formed from the reaction of aromatic dicarboxylic acids with one or more diols have frequently been found useful bas binders in toner powder. See e.g., European Patent Application No. 291,059 to Kubo, et al. and Japanese Patent Application No. 61,176,948. In some cases, the aromatic polyester is formed from fused ring aromatic dicarboxylic acids as in U.S. Pat. No. 4,049,447 to Azar, et al., U.S. Pat. No. 4,246,332 to Tanaka et. al., and European Patent Application No. 333,498 to Takyu, et al.
In producing electrophotographic color images, toner powders containing a high molecular weight polyester having a glass transition temperature of 50 to 100.degree. C. and a weight average molecular weight of 30,000 to 100,000, preferably 55,000 to 65,000, have been utilized. Such molecular weights are well above the critical molecular weight for chain entanglement of that polyester. Although these high molecular weight polyesters are excellent at minimizing offset, they have undesirably high viscosities.
Instead of utilizing a single polyester, toners often employ polyester blends. Such polyester blends are disclosed in U.S. Pat. No. 3,965,021 to Clemens, et al., U.S. Pat. No. 4,795,689 to Matsubara, et al., U.S. Pat. No. 4,863,824 to Uchida, et al., Japanese Patent Application No. 56-065,146, Japanese Patent Application No. 62-299,859, Japanese Patent No. 1,015,755, and Japanese Patent Application No. 61-176,946.
High molecular weight polyesters with desirable offset properties but undesirable viscosities have been blended with a lower molecular weight form of the same polyester to improve the toner's viscosity and gloss properties. Generally, however, the lower molecular weight polyester has a molecular weight at or above the critical molecular weight for chain entanglement of that polymer. As a result, such low molecular weight polymers are still too high in viscosity to improve the viscosity and gloss properties of the blend over that of the high molecular polyester by itself.